This invention relates to audio signal processing and, in particular, to a circuit that uses spectral subtraction for reducing noise.
As used herein, “telephone” is a generic term for a communication device that utilizes, directly or indirectly, a dial tone from a licensed service provider. As such, “telephone” includes desk telephones (see FIG. 1), cordless telephones (see FIG. 2), speaker phones (see FIG. 3), hands free kits (see FIG. 4), and cellular telephones (see FIG. 5), among others. For the sake of simplicity, the invention is described in the context of telephones but has broader utility; e.g. communication devices that do not utilize a dial tone, such as radio frequency transceivers or intercoms.
There are many sources of noise in a telephone system. Some noise is acoustic in origin while the source of other noise is electronic, the telephone network, for example. As used herein, “noise” refers to any unwanted sound, whether or not the unwanted sound is periodic, purely random, or somewhere in-between. As such, noise includes background music, voices of people other than the desired speaker, tire noise, wind noise, and so on. Automobiles can be especially noisy environments, which makes the invention particularly useful for hands free kits.
As broadly defined, noise could include an echo of the speaker's voice. However, echo cancellation is separately treated in a telephone system and involves a comparison of the signals in two channels. This invention relates to noise suppression, which means that the apparatus operates in a single channel and in real time; i.e. one is not calculating delays as in echo cancellation.
While not universally followed, the prior art generally associates noise “suppression” with subtraction and noise “reduction” with attenuation. As used herein, noise suppression includes subtraction of one signal from another to decrease the amount of noise.
Those of skill in the art recognize that, once an analog signal is converted to digital form, all subsequent operations can take place in one or more suitably programmed microprocessors. Use of the word “signal”, for example, does not necessarily mean either an analog signal or a digital signal. Data in memory, even a single bit, can be a signal.
“Efficiency” in a programming sense is the number of instructions required to perform a function. Few instructions are better or more efficient than many instructions. In languages other than machine (assembly) language, a line of code may involve hundreds of instructions. As used herein, “efficiency” relates to machine language instructions, not lines of code, because the number of instructions that can be executed per unit time determines how long it takes to perform an operation or to perform some function.
A “Bark band” or “Bark scale” refers to a generally accepted model of human hearing in which the human auditory system is analogous to a series of bandpass filters. The bandwidth of these filters increases with frequency and the precision of frequency perception decreases with increasing frequency. Several slightly different formulae are known for calculating the bands. The Bark scale includes twenty-four bands, of which only the lower eighteen bands are used in the invention because the bandwidth of a telephone system is narrower than the full range of normal human hearing. Other bands and bandwidths could be used instead for implementing the invention in other applications.
In the prior art, estimating noise power is computationally intensive, requiring either rapid calculation or sufficient time to complete a calculation. Rapid calculation requires high clock rates and more electrical power than desired, particularly in battery operated devices. Taking too much time for a calculation can lead to errors because the input signal has changed significantly during calculation.
In view of the foregoing, it is therefore an object of the invention to provide a more efficient system for noise suppression in a telephone and other communication devices.
Another object of the invention is to provide an efficient system for noise suppression that performs as well as or better than systems in the prior art.
A further object of the invention is to provide a noise suppression circuit that introduces less distortion than circuits of the prior art.